Anodic aluminum material and articles and composite articles comprising the material

ABSTRACT

AN ARTICLE COMPRISING AN ANODE MATERIAL COMPOSED OF A SOLUTION HEAT TREATABLE AND PRECIPITATION HARDENABLE ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF 2 TO 8% ZINC, 0.5 TO 2% MAGNESIUM, 0.3 TO 2% SILICON, THE SILICON CONTENT AMOUNTING TO AT LEAST 0.45 TIMES THE MAGNESIUM CONTENT, THE BALANCE BEING ALUMINUM AND INCIDENTAL ELEMENTS AND A MAXIMUM OF 0.25% COPPER AND A MAXIMUM OF 0.25% CHROMIUM, AS IMPURITIES, THE MEMBER EXHIBITING A STABLE SOLUTION POTENTIAL UPON EXPOSURE TO VARYING THERMAL CONDITIONS. ALSO CONTEMPLATED IS A COMPOSITE HAVING A CLADDING COMPOSED OF THE MATERIAL AND A HEAT TREATABLE ALUMINUM ALLOY CORE.

United States Patent AN ODIC ALUMINUM MATERIAL AND ARTICLES ANDCOMPOSITE ARTICLES COMPRISING THE MATERIAL Robert H. Brown, NatronaHeights, William A. Anderson, Pittsburgh, and William King, LowerBurrell, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa.No Drawing. Filed Apr. 21, 1969, Ser. No. 818,145

Int. Cl. B23p 3/02 US. Cl. 29197.5 9 Claims ABSTRACT OF THE DISCLOSUREAn article comprising an anode material composed of a solution heattreatable and precipitation hardenable aluminum base alloy consistingessentially of 2 to 8% zinc, 0.5 to 2% magnesium, 0.3 to 2% silicon, thesilicon content amounting to at least 0.45 times the magnesium content,the balance being aluminum and incidental elements and a maximum of0.25% copper and a maximum of 0.25% chromium, as impurities, the memberexhibiting a stable solution potential upon exposure to varying thermalconditions. Also contemplated is a composite having a cladding composedof the material and a heat treatable aluminum alloy core.

BACKGROUND OF THE INVENTION There is an increasing need for aluminummaterials having a relatively high anodic solution potential togetherwith substantial strength. Such materials are useful to provide cathodicprotection to structural members fashioned from other aluminousmaterials and are often more useful if they can be imparted withsubstantial strength so as to be suitable for bearing loads. One suchapplication is where the anodic material serves as a protective claddingin a composite having a heat treatable core. Accordingly, aluminum alloymaterials have been proposed which have relatively anodic electrolyticpotentials and which are precipitation hardenable, by which is meantartificially ageable, which materials are useful in providing anodeswhich can bear loads. However, these materials are marked by variationsin solution potential upon thermal exposures such as encountered inprecipitation hardening treatments and even such as encountered inactual service. For example, an aluminum alloy containing zinc andmagnesium and generally having a suitable anodic electrolytic potentialwill be observed to exhibit widely varying solution potential valuesvarying over a range of up to 100 mv. and even more in response tovarying aging or other thermal exposure conditions. In the case of acomposite, the desired extent of the difference in solution potentialbetween cladding and core can vary from one application to anotheralthough a difference of about 100 mv. is often desired. However, insuch a system, it is quite apparent that where the solution potential ofthe cladding can vary up to 100 mv. because of differing thermalexposures, it becomes quite difiicult to assure any desired degree ofcathodic protection. In addition, some core compositions require thatthe cladding solution potential be much greater than the core potential,there being some applications where a difference of 175 mv. is notsufficient. When it is noted that the total span in solution potentialbracketing aluminum and all its commercial alloys amounts to about only300 mv., it is quite apparent that fluctuations approaching 100 mv. canbe extremely troublesome. As an example of thermal conditionsencountered in service, in an automobile radiator an aluminum membercould easily encounter a temperature considerably exceeding 170 F. andthe precipitation 3,674,448 Patented July 4, 1972 ice hardenablealuminum anode materials proposed to date exhibit differences inelectrolytic potential upon such exposures which are often consideredunacceptably wide. Thus, the extent of the variations in potentialrenders it ditlicult to properly balance a dual alloy system from thestandpoint of cathodic protection and an anodic material having athermally stable solution potential would be highly desirable.

STATEMENT OF THE INVENTION In accordance with the invention, there isprovided a strong solution heat treatable and precipitation hardenablealuminum alloy article which exhibits a very stable solution potentialwhich will not vary by more than 50 mv. regardless of any variations inthermal exposure which might occur from situation to situation duringthermal treatments such as precipitation hardening treatments or duringservice.

The foregoing advantages are achieved by providing an article composedof an aluminum base alloy consisting essentially of 2 to 8% zinc, 0.5 to2% magnesium, 0.3 to 2% silicon, the silicon content amounting to atleast 0.45 times the magnesium content, the balance being aluminum andincidental elements and a maximum of 0.25% copper and a maximum of 0.25%chromium, as impurities. The percentages are all by weight. Articlesfashioned from this alloy will exhibit upon exposure to varying thermalconditions a very stable electrolytic potential which does not vary bymore than 50 mv. even though the temperature, time at temperature andother factors may be varied from one instance to another. Because thealloy is precipitation hardenable, it exhibits substantial strengthtypified by strength levels of 35 to over 50 k.s.i. tensile and 25 toover 40 k.s.i. yield strength. In practicing the invention, it isessential that, as indicated above, the silicon content must amount toat least 0.45 times the magnesium content. This assures that the desiredstable solution potential will be achieved in each case. In order tofurther assure such and to further improve the strength of the material,it is preferred that the silicon content amount to 1 to 2 times themagnesium content. From the standpoint of improved strength, it isfurther preferred that manganese be included in an amount of 0.1 to0.75%. Where higher levels of electrolytic potential are desired, forinstance, levels consistently over 980 mv., the zinc content should be3.5 to 8%.

In the case of a composite, a core material most highly suited is one ofthe alloy system usually designated the Al-Zn-Mg-Cu system. This type ofalloy carries the Aluminum Association Registration designation 7XXX andthese alloys contain up to about 3 to 9% Zn, 1 to 3% Mg, copper in anamount of up to 3% together with additions of Cr, Mn and other elements.Specific examples of these alloys include the following nominalcompositions:

7075: 5.6 Zn, 2.5 Mg, 1.6 Cu, 0.3 Cr, bal. A1 7079: 4.3 Zn, 3.3 Mg, 0.6Cu, 0.2 Mn, 0.2 Cr, bal. Al

The 7XXX type of alloy is known to exhibit high strength after solutionheat treatment and precipitation hardening although it is also knownthat in some cases it is advisable to provide cathodic protection bymeans of an anodic cladding to which purpose the invention is especiallysuited. However, while the invention contemplates use of the improvedanodic material as a cladding on a 7XXX aluminum alloy, in a broadersense, the improved anodic material should be useful with other heattreatable aluminimum cores and it is not necessarily intended to limitthe invention to the use of a 7XXX core. In any event, the solutionpotential of the core will most often vary from about 780 to about 850mv. after precipitation hardening and the improved material providessuitable cathodic protection for such.

The articles can be manufactured by more or less stand ard techniques.Accordingly, an alloy of proper composition is cast into an ingot as bycontinuous casting and the ingot broken down by hot or cold working orboth. Suitable working operations would include rolling, extruding,drawing and the like.

In the case of a composite which may be sheet, the core and claddingmaterials are separately processed as sheet or plate and then hot rolledtogether to provide a metallurgically bonded composite laminate. In suchrolled composites, the cladding may suitably constitute about 3 to ofthe total composite thickness. In some cases, especially where the corecontains magnesium, it may be desirable to provide an interliner,intermediate the core and cladding and containing little or nomagnesium, to assure a quality bond. Such has proven useful in producingcomposites utilizing the improved anode material with 7XXX type cores.

Articles produced from the improved material may have their strengthimproved by solution heat treatment and precipitation hardening.Solution heat treatment may be eifected by heating to a temperature of825 F., or preferably a somewhat higher temperature of about 870 to 970F. or more. Where the highest strength is desired, the solution heattreatment temperature should be at least 950 F. The article ismaintained at solution heat treating temperature for a period of timesufficient to take into solution substantially all the soluble alloyconstituents. This time varies from case to case depending on therelative size of the article and the heating equipment employed and willgenerally vary from a matter of several minutes to several hours. Wherethe article is a composite article, the solution heat treatment time andtemperature may be fixed by the particular core material utilized andthe improved materials solution potential stability is especiallydesirable in such a case.

After the solution heat treatment, the article is quenched as by coldwater immersion and then subjected to a precipitation hardeningtreatment wherein it is heated to a temperature of up to 400 F., butwhich generally ranges from 200' to 375 F., for varying durationsdepending on the exact physical and corrosion properties needed. In thecase of a composite, the precipitation hardening conditions may be fixedby the particular core material employed and again the stable solutionpotential of the improved anodic material is especially useful. Asindicated earlier, prior heat treatable anode compositions normallyexhibit widely varying electrolytic potential levels in response tovarying solution heat treatment and precipitation hardening treatmentswhereas the improved material exhibits highly stable solution potentiallevels. This permits the selection of thermal treatment conditionswithout regard for any sensitivity on the part of the cladding.

In order to demonstrate the improvement derived from practicing theinvention, the following examples proceed:

Example 1 TABLE I {Anodic alloy compositions] Composition:

In the above list, Composition A is clearly outside the practice of theinvention and exemplifies an aluminumzinc-magnesium alloy which, inturn, typifies prior heat treatable aluminum alloys having an anodicelectrolytic potential. Even though the alloy constituents, Zn, Mg andSi, broadly speaking, can be viewed as present within the ranges claimedherein, Composition B is nonetheless outside the invention as the ratioof its Si to Mg content amounts to only 0.38 which is less than the 0.45level set out as essential in the practice of the invention. CompositionC is within the practice of the invention with respect to the Zn, Mg andSi contents along with the silicon to magnesium ratio which is slightlyover 0.62. Several plates of each composition were solution heat treatedfor two hours at two different temperatures. The plates were quenched byimmersion in cold water and then precipitation hardened under varyingconditions according to Table II. Oneeighth inch diameter shorttransverse, that is, across the 2 inch plate thickness, specimens wereremoved and tested for tensile properties. In addition, electrolyticpotential measurements were made by measuring the specimens against a0.1 N calomel electrode in 1.0 N NaCl plus 0.3% H 0 electrolyte, aprocedure commonly used in the aluminum alloy art for such measurements.The results of these measurements are set forth in Tables III to V whichset out the tensile and yield strength (TS. and Y.S.) together with theelongation in 2 inches and the solution potential (Pot) for each of thecompositions in Table I under the seventeen diife'rent conditions ofthermal treatment described in Table II.

Referring to Table III and Composition A, it can be seen that thedifferent thermal treatments result in widely varying strengthproperties. It is highly sigm'ficant that for the solution heattreatment at 870, the potential measurement spread was over a range of93 mv. For the other solution heat treatment temperature, 970 F., thespread was a little less at 74 mv. but still is considered excessivelyhigh. Similar results were encountered with the Composition B specimensas is seen in Table IV. Turning now to Table V and the Composition Cresults, it can again be seen that some variation in strength is causedby differing thermal treatments. However, it is readily apparent thatthe potential measurements did not vary appreciably. On the contrary,they were very stable. At a solution heat treatment of 870 F., the totalpotential spread amounted to only 10 mv. regardless of the precipitationhardening treatment. Equally significant, the results for solution heattreatment temperature of 970 F. indicate a total variation of only 13mv. The total variation of all the results for Composition C amounts toonly 29 mv. whereas for Compositions A and B, the total variationamounts to over 3 times that much, i.e., respectively, 93 and 96 mv.Where it is deemed essential for cathodic protection that TABLE II[Solution heat treatment and precipitation hardening conditions] Sol.H.T., hr. at F. Prec. hdn., hr. at F.

15.-.. do 24 at 340: 16--.- do 3 at 250 plus 3 at 325". 17 do a at 250plus 15 at 325.

TABLE 111 [Strength and potential for Comp. A (0.08 81)] T.S. Y.S.Elong. Pot. S read (K s.i.) (K s.i.) (percent) (mv.) mv.)

Condition No.:

TABLE-IV [Strength and potential for Comp. B (0.46 Si) T.S. Y.S. Elong.Pot. Spread (K s.i.) (K s.i.) (percent) (mv.) (mv.)

Condition Na:

TABLE V [Strength and potential for Comp. C (0.76 Si) 'I.S. Y.S. Elong.Pot. Spread (K s.i.) (K s.i.) (percent) (mv.) (mv.)

Condition No.:

the anode potential be 100 mv. more anodic than the material beingprotected, this potential diiference being a common value often chosenin the industry, it can be seen that variations of 93 to 96 mv. canrender positive assurance of this difference quite difiicult. It isapparent that the comparatively minuscule potential variation associatedwith the practice of the invention minimizes this problem.

Referring again to the tables, particularly Table V, it is apparent thatthe improved material exhibits substantial strength especially whensolution heat treated at a temperature of over 950 F. In such cases, thetensile strength is over 45 K.s.i. regardless of the thermal treatment.Composite laminates, which included the improved anodic material ascladding, exhibited substantially higher strength than similar laminatesfeaturing a non-heat treatable cladding and this strength improvement isrealized without substantial variations in the solution potential of thecladding in response to varying thermal treatments thus rendering theimproved material very useful as an anodic cladding layer.

What is claimed is:

1. An aluminum anode composed of a solution heat treatable andprecipitation hardenable aluminum base alloy consisting essentially of 2to 8% zinc, 0.5 to 2% magnesium, 0.3 to 2% silicon, the silicon contentamounting to at least 0.45 times the magnesium content, the balancebeing aluminum and incidental elements and a maximum of 0.25% copper anda maximum of 0.25% chromium, as impurities, said alloy articleexhibiting, when subjected to a thermal exposure, a stable solutionpotential such that it does not vary by more than 50 mv. irrespective ofthe conditions of the thermal exposure.

2. The article according to claim 1 wherein the alloy also contains 0.1to 0.75% manganese.

3. The article according to claim 1 wherein the silicon content amountsto l to 2 times the magnesium content.

4. The article according to claim 1 wherein the zinc content is 3.5 to8%.

5. An article comprising a clad composite having a core and a cladding,said cladding being anodic to said core thereby to cathodically protectthe core, said core and cladding each being composed of a heat treatableand precipitation hardenable aluminum base alloy, said cladding beingcomposed of an aluminum base alloy consisting essentially of 2 to 8%zinc, 0.5 to 2% magnesium, 0.3 to 2% silicon, the silicon contentamounting to at least 0.45 times the magnesium content, the balancebeing aluminum and incidental elements and impurities with a maximum of0.25 copper and a maximum of 0.25 chromium as impurities, said corebeing composed of an aluminum base alloy consisting essentially of 3 to9% zinc, 0.5 to 3% magnesium, and copper present in an amount of up to3%, the balance being aluminum and incidental elements and impurities,said core and cladding being adapted to common solution heat treatmentand precipitation hardening treatments, said core exhibiting, after saidtreatments, a solution potential of from about 780 to about 850 mv. andsaid cladding exhibiting, after said treatments, a stable solutionpotential which does not vary by more than 50 mv. irrespective of theparticular time and temperature employed in said treatments.

6. An article comprising a clad composite having a core and a cladding,said cladding being anodic to said core thereby to cathodically protectthe core, said core and cladding each being composed of a heat treatableand precipitation hardenable aluminum base alloys, said cladding beingcomposed of an aluminum base alloy consisting essentially of 2 to 8%zinc, 0.5 to 2% magnesium, 0.3 to 2% silicon, the silicon contentamounting to at least 0.45 times the magnesium content, the balancebeing aluminum and incidental elements and impurities with a maximum of0.25% copper and a maximum of 0.25% chromium as impurities, said coreand cladding being adapted to common solution heat treatment andprecipitation hardening treatments, said core exhibiting, after saidtreatments, a solution potential of from about 780 to about 850 mv. andsaid cladding exhibiting, after said treatments, a stable solutionpotential which does not vary by more than 50 mv. irrespective of theparticular time and temperature employed in said treatments.

7. The article according to claim 5 wherein the cladding alloyadditionally contains 0.1 to 0.75% manganese.

8. The article according to claim 5 wherein the cladding alloy siliconcontent amounts to 1 to 2 times its magnesium content.

7 8 9. The article according to claim 5 wherein the zinc OTHERREFERENCES Went the claddmg aHOY Metal Selector, 1953 ed., Steel, Oct.14, 1963, pp.

References Cited UNITED STATES PATENTS 2,742,688 4/1956 Nock, Jr.29-1975 5 L. DEWAYNE RUTLEDGE, Primary Examiner J. M. DAVIS, AssistantExaminer 3,287,185 11/1966 Nachet et a1. 75-141 X US. Cl. X.R. 3,342,5659/ 1967 Munday et a1. 29-197.5 X 75146, 147

